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{{Other uses}}
is a hard object within the soft tissue (specifically the [[mantle (mollusc)|mantle]]) of a living [[animal shell|shell]]ed [[mollusk]]. Just like the shell of a clam, a pearl is composed of [[calcium carbonate]] in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes ([[baroque pearl]]s) occur. The finest quality natural pearls have been highly valued as [[gemstone]]s and objects of [[beauty]] for many centuries. Because of this, ''pearl'' has become a [[metaphor]] for something rare, fine, admirable and valuable.▼
{{Use American English|date=October 2015}}
{{Use mdy dates|date=October 2015}}
{{Infobox mineral
| name = Pearl
| boxwidth =
| boxbgcolor =
| image = Various pearls.jpg
| imagesize = 210px
| alt =
| caption = Various pearls
| category =
| formula = CaCO<sub>3</sub>
| strunz =
| dana =
| symmetry =
| unit cell =
| molweight =
| color = white, pink, silver, cream, brown, green, blue, black, yellow, and purple
| colour =
| habit =
| system =
| twinning =
| cleavage =none
| fracture =
| tenacity =
| mohs =2.5–4.5
| luster =
| streak = white
| diaphaneity =
| gravity = 2.60–2.85
| density =
| polish =
| opticalprop =
| refractive =
| birefringence =
| pleochroism =
| 2V =
| dispersion = none
| extinction =
| length fast/slow =
| fluorescence = weak, cannot be evaluated. Genuine black p .: Red to reddish River-p.: Strong: pale green
| absorption =
| melt =
| fusibility =
| diagnostic =
| solubility =
| impurities =
| alteration =
| other =
| prop1 =
| prop1text =
| references =
}}
▲
The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as ''natural'' pearls. ''Cultured'' or ''farmed'' pearls from [[pearl oyster]]s and freshwater mussels make up the majority of those currently sold. [[Imitation pearl]]s are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor and is easily distinguished from that of genuine pearls. Pearls have been harvested and cultivated primarily for use in [[jewelry]], but in the past were also used to adorn clothing. They have also been crushed and used in cosmetics, medicines and paint formulations.
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Though millimeter size range is typically the first factor in determining a cultured pearl necklace's value, the momme weight of pearl necklace will allow the buyer to quickly determine if the necklace is properly proportioned. This is especially true when comparing the larger south sea and Tahitian pearl necklaces.
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[[File:White pearl necklace.jpg|thumb|A necklace of white pearls]]
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